Restructure performance tips (JuliaLang#54211)

This PR makes the documentation page on "performance tips" more
accessible and structured.

- Restructure the page into four main sections:
  1. General advice
  2. Type inference
  3. Memory management and arrays
  4. Miscellaneous
- Increase subsection depth of individual items and move them into the
appropriate category
- Try to put the most important items first, and the least important
last
- Rename one header to unify style

**No content has been modified, this is strictly copy-paste and moving
things around**

Author: gdalle